This invention relates generally to imaging members, and more specifically to an improved migration imaging member and processes thereof. In one embodiment, the present invention relates to migration imaging members with an electrically conductive layer, such as a substantially transparent electrically conductive layer or electrode comprised of an ionically conductive film forming polymer. One specific imaging member of the present invention is comprised of a supporting substrate, a substantially transparent electrically conductive polymer, and at least one imaging layer comprised of a film forming polymer, or softenable layer, and electrically photosensitive particles. The imaging members of the present invention may also include a protective overcoating thereon. With further respect to the present invention, the imaging members thereof can be selected generally for electrophotographic imaging processes including information recording and storage, xerography, printing and duplicating processes. Specifically, the imaging members of the present invention can be selected for migration imaging processes, reference U.S. Pat. No. 4,536,457, the disclosure of which is totally incorporated herein by reference, xerography and xeroprinting/duplicating processes. Advantages of the imaging members of the present invention include, for example, a significant reduction in residual background optical density of the imaged members by eliminating the use of costly and disadvantageous metal electrodes such as aluminum, improved adhesion, and the like. With the imaging members of the present invention, the background optical density is reduced, for example, from about 0.3 for prior art imaging members to about 0.08 in some embodiments. Also, with the imaging members of the present invention, reproducible, stable and uniform optical density of the electrically conductive layer can be more easily achieved than when metals such as aluminum are selected, thus enabling improved image quality. Further, the electrically conductive layer selected for the imaging members of the present invention is not subject to undesirable oxidation as is the situation with metals such as aluminum. Also, with some embodiments of the present invention the imaging members possess D.sub.min comparable to conventional silver halide films, and moreover the imaging members of the present invention are more economical in many instances as compared to prior art imaging members in that there is eliminated the costly aluminizing step selected for the supporting substrate of such members, for example, aluminizing can add up to 11 cents per square foot to the cost of the resulting film, while with the selection of the transparent conductive polymers of the present invention as the electrode, the comparable cost is about 1 cent for such polymers. Cost can be an important factor particularly when selecting the imaging member as film intermediates and in printing processes.
Other U.S. patents of interest include No. 4,391,388; 4,426,435; 4,517,271; 4,407,918; 4,518,668; 4,520,089; 4,533,611 and No. 4,536,458. The aforementioned patents and No. 4,536,457 mentioned herein were located as a result of a patentability search.
Migration imaging systems capable of producing high quality images of high optical contrast density, continuous tone and high resolution are known references, for example U.S. Pat. Nos. 3,909,262 and 3,975,195, the disclosures of which are totally incorporated herein by reference. Other similar imaging members are illustrated in U.S. Pat. No. 4,536,457, the disclosure of which is totally incorporated herein by reference, including the background of the invention. In a typical embodiment of these migration imaging systems, a migration imaging member comprising an aluminized substrate, reference the U.S. Pat. No. 4,536,457, a layer of softenable material and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material is originally in the form of a fracturable layer contiguous to the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer.
Various known means for developing the latent images formed with migration imaging systems may be selected including those illustrated in U.S. Pat. No. 4,536,457. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by aluminized polyester substrate, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. In contrast, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density of about 0.2, which is essentially due to the aluminized polyester substrate. Therefore, the image sense of the developed image is sign-reversed, that is positive to negative or vice versa. Various methods and materials, and combinations thereof have previously been used to avoid such unfixed migration images.
In the heat, or vapor softening developing modes, the migration marking material in the light struck region disperses in the depth of the softenable layer after development and this region exhibits a D.sub.min which is typically in the range of from about 0.6 to about 0.7. This relatively high D.sub.min is believed to be a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. Also, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, that is a monolayer. This region exhibits a maximum optical density (D.sub.max) which is typically in the range of from about 1.8 to about 1.9. Therefore, the image sense of the heat or vapor developed images is sign retained, that is positive-to-positive or negative-to-negative. Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be located in U.S. Pat. No. 3,795,512.
For many imaging applications, it is desirable to generate negative images from a positive original or positive images from a negative original, that is optically sign-reversed imaging, preferably with low minimum optical density (D.sub.min). Although the meniscus or solvent wash away development methods permit optically sign-reversed images with low minimum optical density, they involve removal of materials from the migration imaging member leaving the migration image largely or totally unprotected from abrasion. Further, although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step is impractically costly and inconvenient for the end users. Also, disposal of the effluents washed from the migration imaging member during development is required. While heat or vapor development methods are preferred because they are rapid, essentially dry and produce no liquid effluents, the image sense of the heat or vapor developed images is optically sign-retaining and the minimum optical density is relatively high, for example from about 0.6 to about 0.7.
The background portions of an imaged member may sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and the softenable layer softened by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically from about 0.7 to 0.9 after vapor exposure compared with an initial value of about 1.8 to 1.9. In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically from about 0.35 to 0.65. Alternatively, the migration image may be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system and the heat or vapor development techniques, the softenable layer remains substantially intact after development with the image being self-fixed as the marking material particles are trapped within the softenable layer. Although the minimum optical density (D.sub.min) of images using such techniques is much reduced, there is generally a concurrent drastic reduction in the maximum optical density (D.sub.max) (since these areas consist of marking material particles which have migrated substantially in depth) and consequently the contrast density (D.sub.max -D.sub.min) is also low.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the film and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transporation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in U.S. Pat. No. 3,909,262, the disclosure of which is totally incorporated herein by reference. However, since the migration imaging mechanisms for each development method are different and because they depend on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles, application of an overcoat to the softenable layer often causes changes in the delicate balance of these processes, and results in degraded photographic characteristics compared with the nonovercoated migration imaging member. Notably, the photographic contrast density is degraded. Recently, improvements in migration imaging members and processes for forming images on these migration imaging members have been achieved. These improved migration imaging members and processes are described in U.S. Pat. Nos. 4,536,458 and 4,536,457, the disclosures of which are totally incorporated herein by reference.
In the aforesaid U.S. Pat. No. 4,536,457, there is disclosed, for example, a process wherein a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate and a charge transport molecule, (for example the imaging member described in U.S. Pat. No. 4,536,458) is uniformly charged and exposed to activating radiation in an imagewise pattern. The resistance to migration of marking material in the softenable layer is thereafter decreased sufficiently by the application of solvent vapor to allow the light exposed particles to retain a slight net charge which allows only slight agglomeration, coalescence, and/or slight migration in depth of marking material towards the substrate in image configuration, and the resistance to migration of marking material in the softenable layer is further decreased sufficiently by heating to allow nonexposed marking material to substantially agglomerate and coalesce. This development process is essentially dry and involves no material removal or addition to the imaging member. The image sense of the developed images is optically sign-reversed. The aforementioned migration imaging member utilizes metallized polyester film, such as aluminized polyester film, as the substrate. The D.sub.min for the developed images is typically in the range of 0.25 to 0.35 and the optical contrast density (D.sub.max -D.sub.min) is typically 0.8 to 1.3, depending on whether an overcoating layer is used to overcoat the migration imaging member.
Also, in U.S. Pat. No. 4,536,458, there is illustrated a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate and a charge transport molecule. The migration imaging member is electrostatically charged, exposed to activating radiation in an imagewise pattern and developed by decreasing the resistance to migration, by exposure either to solvent vapor or to heat, of marking material in depth in the softenable layer at least sufficient to allow migration of marking material whereby marking material migrates toward the substrate in image configuration. This migration imaging member utilizes, for example, an aluminized polyester film as the substrate. The D.sub.min for this imaged member is typically in the range of 0.6 to 0.7 and the optical contrast density (D.sub.max -D.sub.min) is typically 0.9 to 1.2.
There are many disadvantages associated with these prior art imaging members. Most notably, many of the aforesaid prior art migration imaging members produce images which exhibit relatively high minimum optical density (D.sub.min). The relatively high minimum optical density renders the imaging members unsuitable for use in many commercial micrographic equipment such as viewers, printers and duplicators. For example, when the imaged member is projected either onto a screen for the purpose of viewing or onto a photoreceptor for the purpose of printing, a relatively high power projection lamp may be required. An increase of 0.3 in the D.sub.min will require doubling of the exposure energy from the projection lamp and consequently greatly increases the amount of heat generated. The large amount of heat generated by high power lamps not only may shorten the service life of the lamp, but even more seriously may cause further softening of the softenable layer resulting in further migration and/or agglomeration of the migration marking material. Thus, the image quality and stability are degraded. Additionally, such uncontrolled and nonuniform heating of the migration imaging member can cause dimensional instability of the imaging member.
To minimize D.sub.min, many prior art migration imaging members utilize a substrate comprising a very thin layer of metal coating, typically from about 0.005 to about 0.01 micrometer thick aluminum, on transparent polyester films. The aluminum coating is deposited by a vacuum-coating process and its thickness and uniformity must be controlled within very tight tolerance limits in order to assure reproducible and uniform optical density over many thousands of square feet of film resulting in greatly increased manufacturing cost of the imaging members. Additionally, it is known that the optical density of the aluminum coating changes with environmental conditions due to the effects of oxidization. Although other more inert metals such as titanium may be selected, these materials are more expensive and/or require more complex and costly manufacturing processes such as sputtering for deposition. Furthermore, the aluminum coating, being 0.005 to 0.01 micrometer thick, can be easily scratched when subjected to rigorous handling conditions, for example, during manufacture, storage or use resulting in loss of electrical continuity and degraded image qualities. Moreover, to minimize manufacturing cost, it is advantageous to produce the electrically conductive layer and the softenable polymer layer in-line, that is in a single coating operation and preferably on a single apparatus without the need for two separate coating processes, for example, by vacuum coating for the metallic conductive layer and solvent coating for the softenable polymer layer as is required for prior art migration imaging members. Another shortcoming is that metallic coatings, such as aluminum, generally exhibit rather poor adhesion to the polyester films. It is often necessary to subject the surface of the polyester films to special treatment processes such as corona treatment to promote adhesion. This results in an additional increase in manufacturing cost of the imaging members. The aforementioned disadvantages are alleviated with the imaging members of the present invention wherein there is avoided a metal substrate such as aluminum.
Therefore, there continues to be a need for improved migration imaging members and processes. Additionally, there is a need for an improved migration imaging member wherein the disadvantages mentioned herein, including the selection of an uneconomical metal such as an aluminum electrode are avoided. There is also a need for an improved migration imaging member which is capable of producing images having a very low D.sub.min close to that of silver-halide images and high contrast density of from about 0.6 to about 1.2 and preferably from about 0.9 to about 1.2, and which exhibits improved interfacial adhesion and greater resistance to the adverse effects of abrasion. Furthermore, there is a need for an improved and less costly migration imaging member in which the conductive electrode and the softenable layer can be produced in a single coating operation and on a single apparatus such as a polymer coater with multiple coating stations. Other embodiments of the present invention and the advantages thereof are illustrated herein.
The expression "softenable" is intended to encompasss any material which can be rendered more permeable thereby enabling particles to migrate therethrough. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
"Fracturable" layer or material refers to any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate. Also, the fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" refers to actual contact, touching, also, near, though not in contact, and adjoining, and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis, the surface of the softenable layer spaced apart from the substrate. "Optically sign-retained" refers to the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member corresponding to the dark and light areas of the image on the original. The expression "optically sign-reversed" refers to the dark areas of the image formed on the migration imaging member that correspond to the light areas of the image on the original and the light areas of the image formed on the migration imaging member correspond to the dark areas of the image on the original. "Optical contrast density" refers to the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" refers to "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. The value of transmission optical density provided includes the optical density of the substrate unless otherwise specified. For prior art migration imaging members, there is usually utilized a very thin layer of a metal such as aluminum, typically 0.005 to 0.01 micrometer thick, coated on a transparent polyester film as the substrate primarily to mininimize the D.sub.min of the imaged member. Also, with these imaging members the transmission optical density of the substrate is typically about 0.2 which is substantially due to the aluminum layer. With the imaging members of the present invention, which utilize in one embodiment an optically transparent electrically conductive polymer coated on a transparent polyester film as the substrate, the transmission optical density of the substrate approaches about zero, thus enabling a substantial reduction, for example, a reduction of 75 percent or more in the residual background optical density of the imaged member.
"Agglomeration" refers to the coming together and adhering of previously substantially separate particles without the loss of identity of the particles. "Coalescence" is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the agglomerate towards a shape of lower energy, such as a sphere.